In one of the threads here someone was working out the percentages, but as i recall, if there's more than 2 or three spaces between the SM and the GS, then overwatch is the best bet. but if the GS is only going to be fired upon once or twice before it can attack, then the melee units have a higher chance of surviving.

don't ever use them to attack, only ever use them in Guard mode. the BA thunder hammer i've found incredibly effective at holding a position. The SW lightening claws are, for me, even better. it's just about using them in the best position.

One thing is for sure (excluding Librarians): Don't attack with melee weapons unless it is mission critical do-or-die. No melee ATTACK is reliable, not even SW Claws, SW Sgt or SW Hammer/Shield (or BA Sgt w H/S - the closest the BA's come to SW melee ability [actually identical to the SW H/S]). The only thing that you can rely on (MUST rely on - it doesn't always work, but odds in your favor in the short term) are Space Wolf Sgts, SW Claws and SW Hammer ON GUARD defending. NOT attacking with no re-roll. The re-roll is a HUGE odds-increaser in your favor. Blood Angels on Guard are not as reliable, except for the Hammer Sgt. If you attack with melee, you will shortly die.

Any other unit, except those listed above, you should be celebrating if they survive being attacked by a a stealer, even if the marine was on Guard.

They seem more useless because even if they have a good percentile chance of surviving you often roll badly on their first attempt at blocking something. Even with a good chance of survival, they just need a single miss and they are over. They are often quite frustrating!